Geotechnical and paving materials such as soil and asphalt are often tested to obtain their engineering properties in axisymmetric triaxial testing cells. The specimen shape is usually a solid right circular cylinder. In rare cases, a hollow cylinder is used. The triaxial cells allow simultaneous pressurizing of the specimen periphery and deviatoric stress loading in one or more directions, usually the axial direction. The pressurizing media include a range of fluids such as air, mineral oil, or water though other fluids may be employed.
Since the test specimens are porous in nature, and the porosity is often structured such that the specimen is permeable to the pressurizing medium therefore, it is necessary to introduce an impermeable seal around the specimen to isolate the mechanical effect of confining stress. In order to allow the specimen to change shape during testing, this impermeable seal must be flexible, usually made of polymers such as latex, nitrile or silicone for testing paving materials and soils, while it may be a metal (e.g. copper) jacket for testing solid rock.
Displacement measurement(s) are usually necessary for computing the desired engineering properties from the stresses and strains (engineering strain is related to displacement through a very simple equation). It is often impossible to use the traditional resistance strain gauge in this application because (a) the specimen sometimes cannot be instrumented with a strain gauge that relies on adhesives, (b) large strains are difficult to precisely measure with typical resistance strain gauges, and (c) the surface void texture causes the strain gauge to be inaccurate. The axis of the cylinder is usually vertical, so the deviatoric loading and the related strains are parallel to this axis. Therefore, the vertical displacement measurements are required for all but the most basic engineering properties e.g., simple material strength does not require the measurement of strain, it only requires measurement of stress. Vertical strains combined with horizontal strains can be used for determining Poisson's ratio and dilation parameters.
The horizontal or radial strain may be measured at a number of points on the surface of the cylinder, or by one or more circumferential measurements which has the advantage of reducing the number of transducers and improving the signal to noise ratio of a given transducer, if it is an analog device, or by other means such as volume change or optical measurements.
U.S. Pat. No. 5,025,668 issued to Sarda et al. has instrumentation which is externally referenced. Such an apparatus has limitations: (1) it is not immune from end effects, and (2) it does not uncouple the vertical from the horizontal displacement. End effects alter strain measurements from the true value because the specimen deforms more like a whiskey barrel than like a right circular cylinder. The end effects are worse (a) when the friction between the specimen end and the loading platen is high, as is the case with many soils and virtually all asphalt materials, (b) when the measurement is taken with a gauge length that spans the whole specimen height from end to end, (c) when the specimen is short, and (d) when the properties at the ends of the specimen are different from the true properties of the specimen in the middle portion of the specimen for example, molded specimens tend to have somewhat different densities and air void properties close to the ends. The end effects often affect the vertical strain measurements more than the radial measurements.
If radial measurements are taken, they should be uncoupled from the vertical movement. Since the radial measurements are usually very small in relation to the vertical on materials having Poisson's ratio smaller than 0.5, friction at the end of the shaft in contact with the specimen can introduce bending and/or binding in the shaft, causing the measured radial deflection to be incorrect.
U.S. Pat. No. 4,579,003 issued to Riley also illustrates instrumentation that is externally referenced. Riley discloses an improvement over the device disclosed by Sarda in that the instrumentation is internal to the triaxial cell, but it is still external to the specimen. At the very small displacements commonly measured with soils and asphalt, any interface between any material other than the material being tested and some other material ,such as a metal or polymer platen, can cause enough deformation under load to totally mask the correct strain measurement. Therefore, measuring between the stages as disclosed by Riley is likely to produce better measurements than Sarda's device.